A number of polyimide and polyamide-imide polymers are known. In general, polymers of this category have been prepared from aromatic reactants. An example is the polyimide prepared from pyromellitic dianhydride and oxydianiline: ##STR2## This aromatic polyimide exhibits an outstanding combination of thermal, mechanical and electrical properties, but is essentially insoluble and infusible. The intermediate poly-(amide-acid) is processible with specialized techniques which involve a final cure, usually by heating, to obtain cyclodehydration to the polyimide.
Recognition of the exceptional thermal stability of the aromatic imide ring led to increased efforts to design polymers which contained the imide function but could be more easily processed. One result was a series of polyamide-imide resins derived from trimellitic anhydride and primary, aromatic diamines, a typical example being the polymer prepared from 4,4'-methylenedianiline as the aromatic diamine; ##STR3## This polymer is soluble in certain polar solvents and has better thermoplastic qualities making it somewhat easier to process. However, its melt characteristics and thermoplastic qualities are generally not good enough to make it suitable for molding applications.
Recent patent literature also contains numerous examples of wire-coating resins prepared from trimellitic anhydride (TMA), pyromellitic anhydride, ethylene glycol, and 4,4'-methylenedianiline. These resins are thermosetting resins of complex structure.
Two of the most promising prior art synthetic routes to polyamide-imides having reasonably good properties are as follows: